U.S. Tries to Isolate Syria's Militant Islamists

WASHINGTON—The Obama administration and its allies are intensifying efforts to weaken Syrian rebels they believe are linked to al Qaeda and other extremist groups, as the battle to overthrow President Bashar al-Assad seems to gain ground, said U.S. and European officials.

The U.S. Treasury Department is preparing to sanction one of the militias fighting against Mr. Assad, called Jabhat al-Nusra, as a foreign terrorist organization....